1. Field of the Invention
The present invention generally relates to a subscriber line system for transmitting signals, such as speech signals, and more particularly to an optical subscriber line system in which subscriber lines are free of maintenance, and terminals can efficiently share spare lines.
2. Description of the Related Art
As communication networks expand, subscriber line circuits become more complicated. There is an increased number of cases where subscriber line circuits in use are required to be changed due to an increase in the number and/or movement of subscribers. There is currently a demand to make the subscriber line circuits free of maintenance and reduce the load of maintenance and operation necessary for changing the subscriber line circuits. In view of the above, it is required that the subscriber lines be formed with optical lines.
In a conventional subscriber line system, 64 kHz signals, each amounting to one telephone line, are individually transferred through respective metallic lines. A plurality of metallic lines, to which subscriber terminals are connected, are grouped and laid to a repeater station. Generally, spare lines are laid in order to cope with an increase in the number of subscribers and movement thereof. In the conventional technique, lines separate from working lines are laid beforehand, as spare lines. Such spare lines are laid so that they connect relays to each other.
FIG. 1 is a block diagram of a conventional configuration in which spare lines are provided. A multiplexer/demultiplexer unit 11 multiplexes subscriber line signals with each other to generate a primary rate output signal, and demultiplexes a primary rate input signal into subscriber line signals. A plurality of relays 13.sub.1 -13.sub.n (where n is an integer) are connected to the multiplexer/demultiplexer 11. A plurality of telephone sets (subscriber terminals) 12 are connected to each of the relays 13.sub.1 -13.sub.n. Five telephone sets 12 are connected to the relay 13.sub.n, and thus five subscriber lines are connected between the multiplexer 11 and the relay 13.sub.n. In the same manner, subscriber lines equal in number to the telephone sets are respectively provided between the multiplexer/demultiplexer unit 11 and each of the relays 13.sub.1 -13.sub.n-1. Further, a spare line is provided per some of the relays 13.sub.1 -13.sub.n. In the configuration shown in FIG. 1, a spare line 14.sub.1 is provided to the relays 13.sub.1 and 13.sub.2, and a common spare line 14.sub.2 is provided to the relays 13.sub.n-1. If a new subscriber terminal 12.sub.1 is connected to the relay 13.sub.1, a connection with the multiplexer/demultiplexer unit 11 through the spare line 14.sub.1 is made, while the spare line 14.sub.1 is disconnected from the relay 13.sub.2.
FIG. 2 shows the multiplexer/demultiplexer unit 11 shown in FIG. 1. The multiplexer/demultiplexer unit 11 comprises a primary rate multiplexer/demultiplexer 16, 24 speech processors (coder/decoders) 17.sub.1 (#1)-17.sub.24 (#24), and a bus 18. The primary rate multiplexer/demultiplexer 16 multiplexes speech signals with each other to thereby generate the primary order output signal, and demultiplexes the primary order input signal into speech signals. Each of the speech processors 17.sub.1 -17.sub.24 encodes speech signals into subscriber line signals, and decodes subscriber line signals into speech signals. The bus 18 connects the primary rate multiplexer/demultiplexer 16 to the speech processors 17.sub.1 -17.sub.24. The bus 18 transfers the subscriber line signals of the 24 channels.
As has been described previously, each time a new subscriber is connected to one of the relays 13.sub.1 -13.sub.n, it is necessary to disconnect, from the spare line, the other relay or relays connected to the above spare line. It is required that the above disconnecting operation be omitted. Further, as the number of subscribers increases and the system is expanded, the load of maintenance increases. Thus it is required that the subscriber lines be free of maintenance.